The Multiverse_Cosmic Origins
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Transcript of The Multiverse_Cosmic Origins
“Our universe may be one of many, physicists say”
The Multiverse
Victor-Andrei Bodiut, S2574608
Coordinator: Prof. Dr. Rien van de Weijgaert
Course: Cosmic Origins (Cosmology) – Minor Astronomy through Space and Time
University of Groningen
28-03-2016
Figure 1 - Artist's Impression of the Multiverse
Imagine a warm tropical
lagoon. You are a fish swimming
around in this lagoon. To you and the
other beings that live here, this watery
world is all there is. This is your
universe. You have no knowledge of
anything beyond. Suppose you got a
clue that yours is just one of the huge
number of bodies of water on Earth’s
surface, some like yours, some very
different. And that creatures are
wandering the soil, the land, the air,
deserts, savannas and forests. Could
your fish brain handle that?
When we (humans now) say universe, we think of it as everything there is because of
the Big Bang, around 13.8 billion years ago. However, the Big Bang that created our universe
may not have been a onetime event. Instead, it could have happened again, and again, and
again, an infinite number of times. And just as our Big Bang created our universe, each of
those events would have created other universes, a process that would increase its rate
exponentially in time. From this perspective, there are no reasons to believe that our universe
was the 1st. The chances for that would be 1/infinity, right? If we want then the word universe
to mean everything there is, we should be careful. For that purpose, the concept of
multiverse was born. Mathematical computations show that the multiverse would contain an
infinite number of universes, sometimes called “pocket” or “bubble” universes. Most of these
“pocket” universes would be very boring, without structure or complexity and lifeless, while
some might even resemble our own. By definition, an infinite number of possibilities means
that everything that can happen, will happen in one universe or another. So is there another
Earth, nurturing intelligent life? Is there/was there/will there be another you and me? This is
certainly a mindboggling idea, but also very controversial, igniting a fierce debate in the
scientific community. How seriously should we take this idea?
In the present paper, I am going to address this issue from a cosmological point of
view. Before doing so, I want to start off with a quote from theoretical cosmologist Andreas
Albrecht at the World Science Festival (2014), which describes the progress made in the last
decades in the field: “I want to start by looking back. When I was a grad student, I wanted to
be a particle theorist. I showed up into Paul Steinhardt’s office (leading developer of cyclic
and ekpyrotic cosmology). I thought he was a particle theorist, but he said - Well actually, I’m
doing cosmology now. I had this old-school training in cosmology where there were big
questions, no good theories and no good data and my heart sank when I heard that.
Figure 2 – A Lagoon
Fortunately, Paul knew about the pioneering work that Alan and Andrei had done (Alan Guth
& Andrei Linde), bringing particle physics to the universe. What happened since then I think
it’s one of the most triumphant transformations of a scientific field ever – from having almost
nothing to having an abundance of riches”.
Cosmic Inflation
The classical Big Bang theory says nothing about what banged, why it banged and
what happened before it banged. When we refer to the traditional Big Bang model, we talk
about the aftermath of the bang, when the matter was already uniformly expanding with the
space-time along with it.
In a pursue to explain what
happened in the instances closer to
the singularity, as well as to account for
several inconsistencies in the original
theory, a group of young physicists
including Andrei Linde, Paul Steinhardt,
Andreas Albrecht and especially Alan
Guth, came up with a twist to the Big
Bang theory in the 1980s. The idea of
cosmic inflation was proposed. It
involves an exponential expansion of
the universe in the very first split-
second after the Big Bang, lasting from
10-36 seconds to ~ 10-32 seconds (see
Figure 3). In the original theory, the universe expands relatively gradual.
The Newtonian gravity (the usual or common notion of gravity that is familiar to us) is
always attractive. However, Einstein’s general relativity allows gravity to be repulsive. Einstein
suggested that if you have enough energy, the gravity becomes an explosive force, pulling
outward, rather than inward.
This gravitational repulsion occurred in the cosmic inflation and was caused by a very peculiar
type of matter. The growth then stopped because the matter was fundamentally unstable
and started decaying (in the sense of radioactive decaying).
Figure 3 - History of Inflationary Universe
Another rather counterintuitive fact is that energy
is not always positive, as previously thought. In fact, the
total amount of energy in the observable universe is
conservative, thus adding up to zero. All the normal
matter – the elementary particles, stars, planets, galaxies,
etc. account for the positive energy, while the
gravitational fields are negatively energized (see Figure
4). This means that, as Alan Guth puts it, “you can create
universes for free and it takes nothing to put into it, it’s
like the perfect free lunch”.
Puzzles of the traditional Big Bang model that the
paradigm of cosmic inflation tried to solve included the “horizon problem”, the “flatness
problem”, the “magnetic monopole problem” and observations of the large scale structure of
the cosmos. Let us discuss them in some more detail.
The “horizon problem” was identified in the late 60s primarily by the American
physicists, Charles Misner. It poses that different regions of the
universe have the same physical characteristics (e.g.
temperature) although there is no “contact” between them, due
to vast distances. This is a paradox, considering that the
exchange of information (e.g. heat, energy) can occur, at most,
at the speed of light. In other words, there simply has not been
enough time for an exchange from one region to the other. In
cosmological terms, it is said that the two regions are outside
each other’s horizon (see Figure 5).
One could try to explain the physical similarity by assuming a
very homogenous early universe. However, all models predict
fluctuations at some point or level. Thus, there is no good basis
for the two regions to look alike. An inflationary perspective solves
this problem by acknowledging that the observable universe before the spurt of expansion
was ~ 1025 or more times smaller in radius than in the traditional theory. Hence, all regions
could have been in contact.
Figure 5 - The Horizon Problem
Figure 4 - Gravitational Field
To put it another way, the observable universe is amazingly uniform on large scales in every direction
we look. We know this by measuring the Cosmic Background Radiation, the thermal radiation left
over from the time of recombination (see Figure 6), with tremendous precision (1 in 100.000). This
cannot be explained by the linear expansion of the classical Big Bang, but can be explained by the
fact that regions were “together enough” before inflation, so that the uniformity could be settled.
Although things are so uniform on large scales, non-uniformities seem to cluster
together on smaller scales. Classically, inflation should produce a perfectly smooth, boring
and lifeless universe, but because of random quantum fluctuations (a higher or lower mass
density here or there), the inflationary models predict a full spectrum of the non-uniformities.
In other words, these quantum fluctuations are the reason for
structure to appear in some places – galaxies, stars, planets, us.
The experimental measurements are in perfect agreement with
the predictions.
The “flatness problem” (or oldness problem) of the
traditional Big Bang model is a fine-tuning cosmological problem.
A fine-tuning problem refers to model parameters that appear to
be tweaked to very particular (or special) values. This becomes
problematic when an underlying mechanism is nonexistent. First
mentioned by American physicist Robert Dicke in the late 60s, the
Figure 6 - Cosmic Background Radiation (CMB)
Figure 7 - The Shape of our Universe Depending
on Ω
fine-tuned parameter in the flatness problem is the density of matter and energy (Ω). Ω
basically determines the curvature of space-time - whether we are living in a spherical,
hyperbolic or flat universe (see Figure 7). The current value of density is very close to the
critical value of a flat universe and was even more so in the past (margin of error of 10-62).
The standard Big Bang model does not provide solid ground for such a nearly isotropic and
flat universe. As cosmologists want to understand and explain the current universe, and not
accept it “as is”, this becomes a problem. Cosmic inflation solves the puzzle by pushing the
universe exponentially towards flatness. At first, the observations of the overall density
differed significantly from what inflation predicted. However, when accounting for dark
energy (~68% of the total energy in the observable universe), the observed and predicted
values matched to a confidence of half-percent.
Lastly, the “magnetic monopole problem” (or the
exotic-relics problem) is a contradiction between what is
hypothesized and what is observed. Stable magnetic
monopoles (see Figure 8), hypothetical elementary particles
in physics that originate in the early hot universe and should
have persisted and become abundant have never been
observed. An inflationary colder period would dilute any
relics, explaining the lack of observational evidence. To be
mentioned here is Sir Martin Rees, the U.K.'s Astronomer
Royal’s comment: "Skeptics about exotic physics might not
be hugely impressed by a theoretical argument to explain the
absence of particles that are themselves only hypothetical. Preventive medicine can readily
seem 100 percent effective against a disease that doesn't exist!"
A step forward – Eternal Chaotic Inflation
In 1983, Andrei Linde developed on the idea of cosmic inflation even further. He saw
our universe as a simple “bubble” growing up in a vastly grander scale – the multiverse. This
is possible due to a vacuum that did not decay to its ground state, as in Guth’s cosmic
inflation model. Linde thought that maybe Einstein wanted too much in his pursue of
explaining why the universe is the same everywhere. Maybe that was not the case after all.
During one conference, Linde gave the example of the universe as a football, with its
characteristic white and black spots. Assume the football undergoes inflation and expands
exponentially. If we happen to have lived on a black spot before the spurt, our entire
observable universe will now be black. On the other side, if we happen to have begun our
journey on a white spot, our universe is now all white. The laws of physics in a black universe
Figure 8 - Magnetic Monopoles
can be somewhat different or very different from the ones in a white universe. In the process,
all combinations of grey universes must have been formed as well.
On the same line of thought, computer
simulations on the consequences of chaotic
inflation show the multiverse as a growing
fractal (see Figure 9). In this grand picture, each
“bubble” is a universe generated by its own Big
Bang. The different colors are meant to show
the different types of universes, governed by
somewhat different physical laws. There still is
one or more fundamental laws operating the
whole fractal, but each “bubble” takes a
different actualization of those laws. The
fundamental laws dictate the variety of local by-
laws that could exist. And so, our universe
would be governed by one manifestation of
these by-laws. Each “bubble” is so incredibly vast
that one cannot hope to reach another “bubble”. The reason for this is that the space in
between universes is thought to be of a higher dimension, unperceivable to our “fish brain”.
Not to add that it would be quite dangerous to visit other universes governed by different
physical laws, as our atoms are held together by our local physical laws – oops!
There are no reasons to believe that the Big Bang generating mechanism will stop. On
the contrary, the rate in which Big Bangs are being produced is increasing exponentially, as
can be seen in the branching of the fractal. As
each “bubble” is expanding exponentially (ours
included) and the number of “bubbles” is
increasing exponentially, the multiverse is infinite.
Another question that follows logically is: is
this fractal the full picture? The multiverse is
described nowadays as having more levels. To
put things in perspective, a next level would imply
a place with fully separated fractals, governed by
completely different physical laws. We’ll step into
that a bit later.
Figure 9 - Linde's Multiverse as Fractal
Figure 10 - Self-reproducing Universe with Two Scalar Fields
A theory of everything? - Quantum Field Theory & String Theory (M-theory)
As Steven Weinberg (American theoretical physicist and Nobel laureate in Physics)
puts it, in the quantum world the fundamental concept is not the particle, but the wave
function, describing all possibilities of existence. Therefore, a form of matter can exist in two
places simultaneously and it is only by an outside observation or intervention that the wave
function collapses and the particle appears to be here or there. The best way to understand
our world is to see the universe as some kind of quantum mechanical superposition of
different possibilities. Without going deeper into a field that might outreach us, let us keep in
mind that quantum field equations are used to explain and predict how the fundamental
particles and the laws of nature behave.
As a next step, string theory replaces the zero-
dimensional point-like particles with one-dimensional extended
objects – strings (see Figure 11). Take a guitar string as a
figurative example. Based on the tension of the string and the
energy it receives, the guitar string will produce musical notes.
Think of these musical notes as excitation modes. Similarly,
from a string theory perspective, the elementary particles can
be thought of as excitation modes of the one-dimensional
strings. Only this time, the average size of a string would be
around the Planck length ~ 10-33.
In an interview with public intellectual Robert Lawrence
Kuhn, Leonard Susskind, one of the fathers of string theory, beautifully describes its
contextual value. What string theory adds to chaotic inflation and the multiverse is something
about the number of types of possible universes built-in the equations. This number is much
larger than the number of atoms in the universe and basically much larger than anything we
can think of. The number 10500 is talked a lot. “By studying the ways in which microscopic
geometries can be combined, at least 10500
have been estimated”. That is not 10500
different “pocket” universes or “bubbles”,
but 10500 types of them, each being
generated again and again. We can bring
chaotic inflation and the string theory
landscape together with a simple
illustration – card decks. String theory tells
us the way the decks will be shuffled – the
order of cards within each deck. Chaotic
inflation is a card factory, creating decks
above decks, each shuffled differently. Ta-
dam!
Figure 11 - Cosmic String Network Simulation
Figure 12 - Rotating Cosmic String Loop on Surrounding Plasma
This is seen as crucial, as it can provide a natural (as
opposed to supernatural) explanation for the anthropic
dilemma of why does our universe have the fundamental
physical constants and the age necessary to foster life,
especially conscious life. As the multiverse is so incredibly vast
and diverse, some of the “bubbles” will happen to have the
right conditions for evolving life, no matter how unlikely.
Assuming a multiverse governed by eternal chaotic inflation
and the string theory landscape, almost all of the “bubbles”
will be sterile, dead and lifeless. Whether the constants of
physics are not quite right, inflation happens too fast, or
electrons are inexistent, all sorts of things can make it go
wrong.
Figure 13 depicts a 6-dimensional Calabi-Yau manifold,
conjecturing the extra 6 dimensions of space-time as described
particularly by superstring theory. Superstring theory basically is meant to be a theory of
everything, explaining all particles and laws of nature as vibrations in supersymmetric strings.
Levels of the multiverse
Cosmologist Max Tegmark distinguishes 4 different types or levels of the multiverse,
each with its particular assumptions and implications. In the existing literature, several other
universe-generating mechanisms have been conceptualized. In general, the more advanced
the levels get, the less scientific basis is involved. The different levels are not mutually
exclusive.
It is a consensus in the scientific community of cosmology nowadays that the
observable universe is not the whole story. There is more
to the physical world than can be seen through the best
telescopes (at least 10 billion light-years away). We are
certainly bounded by the limited speed of light (~ 300.000
km/s) that started travelling ~ 13.8 billion years ago. Thus,
there is a limit to how far our telescopes can see due to
how far the light could get from the Big Bang. Moreover,
the universe is undergoing an accelerated expansion.
There is every reason to expect other galaxies beyond the
visible horizon. Thus, within our own local “pocket”
universe, different regions of the same space-time must
exist. This is Tegmark’s 1st level and a consequence of
chaotic inflation.
Figure 13 - 6D Calabi-Yau Manifold.
Figure 14 - The Hubble Ultra-Deep Field
Linde’s eternal chaotic inflation and the string theory landscape describe Tegmark’s
2nd level multiverse. This is generally referred to as “the multiverse” and
has been presented in more detail above (see also Figure 9 and
Figure 1). The idea has many supporters on both sides and aspects of
the debate between the different views will be addressed shortly.
Tegmark’s level III is a consequence of the strange quantum
world. By taking the quantum wave function as objective reality, the
world splits into an infinity of branches at each instant (e.g. Planck
time ~ 10-43 seconds – the time for a photon to travel a Planck space
~ 10-35 m). This happens in a so-called “Hilbert space” (see Figure 15),
which is very different from our space-time, of infinite dimensions
and abstract. In theory, other universes could be very close (in space-
time) to our own, but dispersed into the intangible Hilbert space. This is
also known as the “many worlds” interpretation of quantum mechanics, and although it has
received some considerable support, it is generally viewed
as more metaphysical than scientific.
Level IV: in Tegmark’s view, whatever a consistent
mathematical system can express must exist is some
kind of universe. "It would seem odd if there were some
basic asymmetry built into math, such that some
equations would be allowed to describe a physical
universe and others would not. So my guess is that
every mathematical structure which mathematicians can
study is on the same footing and describes some kind of
physical universe. I think that the reason that nature is so
well-described by math is because in a very deep sense, nature really is math” (see Figure 16).
As one can guess, this proposal has only a handful of supporters.
Another proposed theory is that
we are currently living in a particular
temporal period of our universe, which
is cyclically expanding and contracting
in Big Bangs and Big Crunches
respectively (see Figure 17). This
mechanism would generate universes
in a sequential and not parallel
manner.
Figure 15 – Hilbert Space
Figure 16 – Mathematical Multiverse
Figure 17 – Cyclical Universe
Higher dimensions of space-time could exist, embedding
completely independent realities. Again, these dimensions could be
in close proximity, but with respect to information transfer and
communications, forever apart.
Lastly, philosopher Robert Nozick has talked about the
principle of fecundity. It states that everything one can conceive,
every single thought and possibility, imagined or real, must exist
somewhere. And so, according to him, infinity is once again the key
to understanding.
One hell of a debate
Several scientists, including some that helped creating the
very theory of inflation in the first place, have come up with counterarguments to the
multiverse theory. Others favor such an explanation. Let us step in a debate between some of
world’s most brilliant minds.
Andreas Albrecht expressed that infinity should not be the way to explain things. “All of
human knowledge will be finite, no matter how big. The concept of infinity can be very useful
in mathematics as an approximation of a very large finite number. Our data is finite, so we
should come up with theories of the universe that work out fine for finite ideas and data. If
you want a round tire for your car, you can use a finite approximation of pi. If you want it a
bit rounder, increase the decimals of pi. The idea is that finite works just fine. The multiverse
theories insist on the concept of infinity to be absolute”.
Pursuing the idea of finiteness, physicist Neil Geoffrey Turok brings about the simplicity
observed in the universe: “I do think in the last 30 years the universe showed to be very
simple and so our theories should be clear, simple and predictive, with compelling results. I
am sad to see that in these years we have seen a proliferation of models, due to a lowering
of standards where we accepted models as legitimate, models which failed to explain the
most significant mysteries about the universe”.
As a counterargument, Andrei Linde gives the example of Steven Weinberg and his pursue
on special relativity: “When we try to think in a different way, when we try to envision a
completely different world, there is a resistance. But the Genie is out of the bottle and is very
hard to put it back. An immense interest is received by the possibility of multiple universes,
the number of people engaged is “exponentially expanding”. On this line of thought, Martin
Rees thinks we might be facing a conceptual leap: “The idea of multiverse could be a
conceptual leap, just like moving from a geocentric to a Copernican universe, or from the
Figure 18 – Fecundity (Oil Painting)
idea that our Milky-Way galaxy is unique to the realization that our galaxy is one of billions.
Kepler thought that the laws governing our solar system are very special”.
Cosmologist and mathematician George Ellis sees the explanation of multiple
universes more of a philosophical explanation, rather than a scientific one: “A scientific theory
should be testable and verifiable. The other dimensions or universes in the multiverse cannot
be tested as we cannot hope to reach them. If we could deduce the laws of physics that
generate our local sub-laws, we could solve this problem, but they are also unverifiable. The
multiverse gives a very plausible explanatory pattern of a scientific nature for the existence of
life in our universe, but still metaphysical because it cannot be tested”.
Martin Rees sees this issue as speculative science, not just metaphysics: “As an example, we
believe that we can talk about the first seconds after the Big Bang, when hydrogen and
helium were made because the laws of nuclear physics that applied then can be tested here
on Earth. In the same way that we think we know what happens inside the Sun from testing
here on Earth. So it could be that we could corroborate the theories referring to multiple Big
Bangs in other ways. Another possibility is that the multiverse model will make some
predictions about this particular manifestation that we live in and we can test these
predictions and maybe refute them. For these reasons, I would regard this subject as part of
science – it is potentially testable, linking in into today’s laboratory physics even and it is
potentially falsifiable. Thus, it is not purely metaphysics, although it has some metaphysical
implications certainly”.
Steven Weinberg talks about the statistical beauty of the theory: “The theory of
multiple universes is not as beautiful as a theory that is very logically constrained, that makes
things look like that is the only way it could be. Still, you cannot reject a theory because it is
not as beautiful as we want it to be. Many beautiful theories tell something about statistical
aspects, and not fundamental behavior of individual components. For example, the theory of
thermodynamics is regarded in consensus as a beautiful theory. However, it does not say
anything about where each particle is in a particular gas. In the same way, there might be a
theory that describes the statistical distribution of the distances between planets and their
parent stars without precisely giving a model of prediction for any particular star. It is still a
beautiful theory. Thus, the multiverse theory could be a statistical theory of multiple universe,
without explaining why each universe is the way it is”.
English physicist Paul Davis makes a rather distinctive point by comparing the
multiverse with a god: "I suppose, for me, the main problem is that what we're trying to do is
explain why the universe is as it is by appealing to something outside of it. In this case, an
infinite number of multiple universes outside of our universe is used as the explanation for
our universe. To me, multiverse explanations are no better than traditional religion, which
appeals to an unseen, unexplained god, a god that is outside of the universe to explain the
universe. Something is amiss”.
When considering the further theoretical and experimental research directions, it comes
down to two fundamental questions that pose a great challenge to 21st century science and
maybe beyond:
1. Was our Big Bang the only one?
2. If there were multiple Big Bangs, were they replicates of each other, or they ended up
being governed by different laws?
Leonard Susskind incorporates a very important aspect of critique on this issue:
“Theoretically, we will explore this land better and better and in the end find approximately
what combinations of the basic elements create a universe like our own. Also, we will better
understand the mathematics of chaotic inflation, make sure they make sense and that the
theory holds together. The critics correctly say that there must be evidential observational
data to it! That’s the hard stuff. We are rapidly coming to an end of the possibility of doing
experiments within a lifetime. Current experiments in particle physics involve 30-50 years of
work. An accelerator that could truly test the things we are interested in would have to be as
big as the Galaxy, using a trillion barrels of oil/second. On another level, astrophysical
observations are coming close to the horizon. We know we cannot see things further than
the horizon. Again, we are coming to an end. We are never going to see other “pocket”
universes, there are outside our horizon, and too far away, outside of our experience. What
we can do experimentally is look into the past and hope to discover that our universe was
born from a bubble nucleation. Thus, observational possibilities are very limited, at least in
the short term”.
As for myself, my opinion is best articulated in Martin Rees’s words: “Well, I feel that the
only appropriate stance is agnosticism because we just don’t know. But I would say that there
are some physicists who I believe conflate what they would like to be the case with what is
actually likely to be the case. Many people would like to be able to write on their T-shirts the
equations that exactly determine the laws of physics that we observe here on Earth. That’s a
worthy goal, it’s wonderful that people are searching for this, but they may fail. It could be
that the laws as we understand them traditionally are just these local environmental accidents
of the aftermath of our particular Big Bang and that there are laws at a much deeper level. I
find that a rather grand and fascinating concept, even though it means we are further from
being able to grasp the final laws”.
References/Bibliography
http://web.stanford.edu/~alinde/
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http://www.physicsoftheuniverse.com/
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http://mentalfloss.com/article/68975/what-multiverse
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waves/
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http://www.kias.re.kr/sub03/sub03_02_10_01.jsp
http://www.ribbonfarm.com/2015/08/20/qft/
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http://news.nationalgeographic.com/news/2014/03/140318-multiverse-inflation-big-bang-
science-space/
http://blogs.scientificamerican.com/life-unbounded/does-a-multiverse-fermi-paradox-
disprove-the-multiverse/
https://www.youtube.com/watch?v=aUW7patpm9s
http://www.closertotruth.com/topics/cosmos/vast-cosmos/multiple-universes-multiverse
https://www.newscientist.com/article/dn25249-multiverse-gets-real-with-glimpse-of-big-
bang-ripples/
https://en.wikipedia.org/wiki/String_theory
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https://en.wikipedia.org/wiki/Flatness_problem
https://en.wikipedia.org/wiki/Robert_H._Dicke
https://en.wikipedia.org/wiki/Magnetic_monopole
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https://en.wikipedia.org/wiki/Multiverse